GNU Emacs Lisp Reference Manual

23.1.1: Making Backup Files

Function: backup-buffer

This function makes a backup of the file visited by the current buffer, if appropriate. It is called by save-buffer before saving the buffer the first time.

Variable: buffer-backed-up

This buffer-local variable indicates whether this buffer's file has been backed up on account of this buffer. If it is non-nil, then the backup file has been written. Otherwise, the file should be backed up when it is next saved (if backups are enabled). This is a permanent local; kill-local-variables does not alter it.

User Option: make-backup-files

This variable determines whether or not to make backup files. If it is non-nil, then Emacs creates a backup of each file when it is saved for the first time---provided that backup-inhibited is nil (see below).

The following example shows how to change the make-backup-files variable only in the `RMAIL' buffer and not elsewhere. Setting it nil stops Emacs from making backups of the `RMAIL' file, which may save disk space. (You would put this code in your `.emacs' file.)

(add-hook 'rmail-mode-hook 
          (function (lambda ()
                      (make-local-variable 
                       'make-backup-files)
                      (setq make-backup-files nil))))

Variable: backup-enable-predicate

This variable's value is a function to be called on certain occasions to decide whether a file should have backup files. The function receives one argument, a file name to consider. If the function returns nil, backups are disabled for that file. Otherwise, the other variables in this section say whether and how to make backups.

The default value is this:

(lambda (name)
  (or (< (length name) 5)
      (not (string-equal "/tmp/"
                         (substring name 0 5)))))

Variable: backup-inhibited

If this variable is non-nil, backups are inhibited. It records the result of testing backup-enable-predicate on the visited file name. It can also coherently be used by other mechanisms that inhibit backups based on which file is visited. For example, VC sets this variable non-nil to prevent making backups for files managed with a version control system.

This is a permanent local, so that changing the major mode does not lose its value. Major modes should not set this variable---they should set make-backup-files instead.